Orientation and GravityOrientation and Gravity
Seth BachelierSeth Bachelier
Vestibular ClassicsVestibular Classics
January 5, 2007January 5, 2007
Space and Spatial ExtensionSpace and Spatial Extension
Orientation – Perception of motion Orientation – Perception of motion and position of an object with respect and position of an object with respect to frame of referenceto frame of reference
Kant - A sensory system propertyKant - A sensory system property• Gathering and Organization of Gathering and Organization of
information from our sensesinformation from our senses
Reference FramesReference Frames
Egocentric – relative to one’s own Egocentric – relative to one’s own body or headbody or head
Allocentric (exocentric) – defined by Allocentric (exocentric) – defined by gravity and landmarks in the gravity and landmarks in the surroundingssurroundings
Egocentric Co-ordinate System
Planes: X-frontal, Y-sagittal, Z-transversal
Judging Object PositionJudging Object Position
Dependent on retinal informationDependent on retinal information• Extra-ocular muscles (efference copies)Extra-ocular muscles (efference copies)
Good for rapid changes in gaze directionGood for rapid changes in gaze direction
• Eye-muscle proprioceptionEye-muscle proprioception Valuable in maintaining resting position of Valuable in maintaining resting position of
the eye in the absence of visual cues the eye in the absence of visual cues (darkness)(darkness)
Visually perceived eye level - VPEL
Subjective straight out from the face – SSFF
VPEL and SSFF in upright position and 30 degree backward tilt
The Tilt Chair
When gravitoinertial force is increased, a shear force acts posteriorly in the plane of the utricle.
This causes an illusion of a backward head tilt reflected in a depression of the VPEL and SSFF relative to the gravitoinertial horizontal.
Gondola Centrifugation
During centrifugation, the long axis (z-axis) is aligned with the resultant force vector from gravity and centrifugal force.
Data was obtained after at least 10 minutes to exclude influence of semicircular canal stimulation.
Results – Tilt Chair (1G)Results – Tilt Chair (1G)
Results - CentrifugeResults - Centrifuge
Discussion - TiltDiscussion - Tilt In upright posture, VPEL and SSFF coincide and are In upright posture, VPEL and SSFF coincide and are
close to GHclose to GH At 30 degree backward tilt, VPEL remains close to GH At 30 degree backward tilt, VPEL remains close to GH
while SSFF remains fixed relative to the headwhile SSFF remains fixed relative to the head The egocentric frame and sense of eye position are not The egocentric frame and sense of eye position are not
influenced by head tilt at 1Ginfluenced by head tilt at 1G There is no vestibulo-ocular response (VOR), it would There is no vestibulo-ocular response (VOR), it would
change the SSFF with respect to the headchange the SSFF with respect to the head Determining VPEL during backward tilt is a combination Determining VPEL during backward tilt is a combination
of reflexive eye movements AND awareness of gravity of reflexive eye movements AND awareness of gravity and compensation of backward head tiltand compensation of backward head tilt
Discussion - CentrifugeDiscussion - Centrifuge SSFF at 2G –SSFF at 2G – 12 degree downward shift 12 degree downward shift
represents a shift in the egocentric framerepresents a shift in the egocentric frame 2 possibilities2 possibilities Otolith signals change the resting position of Otolith signals change the resting position of
the eye and downward gaze is not accounted the eye and downward gaze is not accounted for by efference copies or proprioception, so for by efference copies or proprioception, so when the egocentric frame is changed, both when the egocentric frame is changed, both egocentric and allocentric data are required to egocentric and allocentric data are required to properly determine orientation in spaceproperly determine orientation in space
In hypergravity, abdominal graviceptors can In hypergravity, abdominal graviceptors can be more accurate than the utricle and are be more accurate than the utricle and are perhaps combined with vestibular information perhaps combined with vestibular information to determine SSFF to determine SSFF
Study #2 – Perceiving “Up”Study #2 – Perceiving “Up”
The force of gravity is determined by The force of gravity is determined by vectorially summing several cuesvectorially summing several cues• Touch receptorsTouch receptors• Intrinsic vision cuesIntrinsic vision cues• Extrinsic vision cuesExtrinsic vision cues• Environmental cuesEnvironmental cues
The direction up will oppose the perceived direction of the pull of gravity.
Separating CuesSeparating Cues
Viewing the image through the shroud prevents visual cues of the spacecraft’s orientation.
The image could be aligned with or orthogonal to the body axis.
And the body could be upright or placed on its left side.
ConditionsConditions
Upright with upright visionUpright with upright vision Upright with visually defined top to Upright with visually defined top to
the rightthe right Left side down with upright vision Left side down with upright vision
aligned with gravityaligned with gravity Left side down with vision to the left Left side down with vision to the left
aligned with the body (not run in aligned with the body (not run in flight)flight)
Results – Normal GravityResults – Normal Gravity
This data is the same as control trials made on Earth.
Results - MicrogravityResults - Microgravity
Data is closely aligned with the body axis in all cases.
Similar results in panels 2 and 3 indicate physical cues had no effect on vision.
This is also recognized in the center panel where although vision is to the right, there are twice as many responses opposite the visual field.
Results - HypergravityResults - Hypergravity
Hypergravity Normal Gravity
Comparisons of normal and hypergravity shows that under hypergravity responses are more aligned with the body axis.
DiscussionDiscussion The data under normal gravity conditions fit The data under normal gravity conditions fit
the expected modelthe expected model Under microgravity, ONLY the body vector Under microgravity, ONLY the body vector
generated a perceived direction of up…visual generated a perceived direction of up…visual cues were ignoredcues were ignored
Hypergravity vector did not outweigh the body Hypergravity vector did not outweigh the body vectorvector• 2 explanations2 explanations• Ignoring the visual cues could be a product Ignoring the visual cues could be a product
of the distracting environment, of the distracting environment, experimental conditionsexperimental conditions
• There were only 4 possible choices for the There were only 4 possible choices for the most convex shape, finer gradations could most convex shape, finer gradations could be usedbe used
DiscussionDiscussion Ultimately, under unusual gravity conditions Ultimately, under unusual gravity conditions
and intense distraction, subjects tended to use and intense distraction, subjects tended to use their body as the primary reference frametheir body as the primary reference frame
The need for improved visual cues to rely onThe need for improved visual cues to rely on• larger fieldlarger field• Higher contrastHigher contrast• Stronger orientation cuesStronger orientation cues• More depthMore depth